1) What is Prandtl number?
▶️~It is defined as ratio of momentum diffusivity to thermal diffusivity.
2)What is LMTD? Why it is used?
▶️~The LMTD is a logarithmic average of the temperature difference between the hot and cold streams at each end of the exchanger.
~Temperature difference that takes place across the heat exchanger from entrance to exit is not linear. Therefore we use LMTD.
3) How we can calculate the LMTD when the inlet & outlet temp. of hot fluid and inlet temp. of cold fluid is known?
▶️~Since the heat released by the hot fluid is equal to the heat gained by the cold fluid, the outlet temperature of the cold fluid can be calculated by using the formula Q=m.Cp.∆T
After calculating the outlet temperature of cold fluid, we can determine the lmtd by using the formula
Lmtd= (θ1-θ2) / ln(θ1/θ2)
Where θ1 and θ2 are the differences between temperatures at inlet and outlet respectively.
4) What is effectiveness of heat exchangers?
▶️~It is defined as ratio of actual heat transfer to maximum theoretically possible heat transfer.
5) Is effectiveness of heat exchanger is always less than 1?
▶️~Effectiveness is a dimensionless quantity whose value lies between 0 and 1.
6) In which case effectiveness of heat exchanger is equal to 1?
▶️~doubt.
7) How does a fluid flow in the heat exchanger by itself or by some force?
▶️~It flows by forced convection.
8) What happens to the heat transfer rate if the convective heat transfer is increased? Give expression.
▶️~ Heat transfer rate also increases as convective heat transfer increases.
Q=hA∆T
Where Q is the rate of heat transfer, h is convective heat transfer coefficient, A is the cross sectional area and ∆T is the temperature difference .
As we can see, Q is directly proportional to h, so if we increase the h, Q will also increase.
9) Elaborate designing of heat exchangers.
▶️~given in Shuchen.B. Thakore book (PED) around 12 to 13 steps.
10) How can you Calculate the Heat duty of heat exchangers when you don’t know LMTD?
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